This invention relates to the polymerization of monomers and more particularly, to a method and apparatus for the polymerization of alpha-olefins which provides for the efficient removal of unreacted monomer from the polymeric product.
Polymers, such as polyolefins like polyethylene and polypropylene, are commonly produced by the vapor phase or liquid phase catalytic reaction of an alpha-olefin with or without an inert diluent. The polymeric product exiting the reactor is usually conveyed to a flash chamber wherein a major portion of the unreacted monomer and solvent is removed from the polymer and recycled to the polymerization reactor. The polymeric product, which still contains residual amounts of unreacted monomer and solvent is next transported to a stripping zone, wherein additional amounts of unreacted monomer and solvent are stripped from the polymeric product. This is generally accomplished by heating the polymeric product and/or passing an inert gas stream, such as steam or nitrogen through the polymer, thereby stripping additional amounts of unreacted monomer and solvent from the polymer. The unreacted monomer-solvent-inert gas mixture is then subjected to a separation process, such as distillation or adsorption to remove the inert gas from the unreacted monomer and solvent, and the latter are recycled to the polymerization reactor.
Japanese patent document Kokai Sho 60-79017 (1985) discloses a process for recovering unreacted olefinic monomer from a polymer product being discharged from a polymerization reactor. The invention of this patent document is stated to be an improvement over prior art procedures wherein the polymer from the polymerization reactor is stripped with nitrogen, thereby necessitating removal of the nitrogen by fractionation or adsorption before recycling the recovered monomer to the polymerization reactor. In the process of this patent document, unreacted monomer is stripped from the polymer with the corresponding alkane, i.e. an aliphatic saturated hydrocarbon having the same number of carbon atoms as the olefinic monomer, obtained from an alkene-alkane fractionator. The stripped gas is recycled to the fractionator.
Adsorption processes for the removal of the inert gas from the unreacted monomer-solvent mixtures are often preferred over distillation processes because of lower equipment and energy costs. It can be appreciated however, that the adsorption procedure practiced must be one that does not return inert gas to the polymerization reactor. Accordingly, if the adsorption cycle includes the step of purging the adsorption bed with the non-adsorbed gas stream without subsequently separating the purge gas from the desorbed product, the purged desorbate will contain inert gas, and accordingly, cannot be recycled to the polymerization reactor.
Because of the value of monomers, it is important to recover and recycle as much unreacted monomer as possible. The present invention provides a method and system for recovering substantially all of the unreacted monomer from the polymeric product while minimizing the introduction of inert gas into the polymerization system.